In the field of highway transportation exacting regulations have been laid down as regards to the maximal gross weight which can be carried by vehicles having different numbers of consecutive axles in contact with the highway and different spacings between the axles. These factors can be considered in the design of a vehicle for continuous highway use, and the gross weights allowable are always less than the vehicle itself is capable of carrying. The gross weight is, of course, the payload of the vehicle, and for the transportation operators it is desirable to load each vehicle as fully as its structure--number and spacing of axles--will allow.
A special form of transportation involves the transfer of ready-mix concrete. While it is desirable to haul as much concrete per load as is possible, problems arise when the vehicle reaches the desired site, and must leave the highway and traverse irregular or disturbed terrain. Existence of supernumerary axles, needed to meet highway load requirements, can then result in interference with movement of the vehicle to the exact site of delivery. As pointed out above, the larger number of axles is not needed for truck structure itself, and no load limit applies at the delivery site.